Silver halide thermally developable photographic elements, especially "dry silver" compositions, processed with heat and without liquid development have been known in the art for many years. Such elements generally are a mixture of light insensitive silver salt of an organic acid (e.g. silver behenate), a minor amount of catalytic light sensitive silver halide, and a reducing agent for the silver source.
The light sensitive silver halide is in catalytic proximity to the light insensitive silver salt such that the latent image (silver specks) formed by the irradiation of the silver halide serves as a catalyst nucleus for the oxidation-reduction reaction of the organic silver salt with the reducing agent when heated above 80.degree. C. Such media are described in U.S. Pat. Nos. 3,457,075; 3,839,049 and 4,260,676. Toning agents can be incorporated to improve the color of the silver image of photothermographic emulsions as described in U.S. Pat. Nos. 3,846,136; 3,994,732 and 4,021,249. Various methods for producing dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747 and Research Disclosure 29963.
These thermally developable photographic elements in many cases lack desired sensitivity in the blue region of the spectrum, and, accordingly, it is desirable to increase the sensitivity of the emulsions to blue light. It is known that spectral sensitizing dyes can be employed in thermally developable photographic elements to increase spectral sensitivity. It is known, for example from U.S. Pat. Nos. 3,457,075 and 3,761,729, that merocyanine dyes spectrally sensitize thermally developable photographic materials. Polymethine dyes, including merocyanine dyes, are described in GB patents 1,367,417 for spectrally sensitizing heat developable photographic materials free from light sensitive silver salts and containing substantially light insensitive silver salts. Quinoline dyes are described in GB patent 1,409,009 to spectrally sensitize heat developable light-insensitive materials. Merocyanine dyes having at most one sulfo group are described in GB patent 1,466,201 for spectrally sensitizing thermally developable photographic materials.
A common problem that exists with heat developable photographic systems when spectrally sensitized is the instability of the material in the time during storage. In many cases, spectral sensitizing dyes may provide reduced sensitivity and higher minimum density (fog) when stored for one or more months before processing.
Accordingly, there has been a continuing need for thermally developable light-sensitive elements which have increased sensitivity, especially in the blue region of the spectrum, and provide stability during storage before processing.